add controls to already generated meta rig
The weights are properties of the vertex groups, so they belong to the mesh object, no matter which rig you use. They get overwritten only if you perform an automatic weighting or a "with empty group" option. 

So you can create the new rig and parent it to the mesh using the simple "Armature deform" option.

The names of the new rig bones should be the same as the old, so everithing should work fine (except for the new fingers bones, of course).

Later you can also perform an automtic weghting limited to the new fingers bones, to have a good starting point to be refined with weight painting or direct weight assign.